1. Field of the Invention
This invention relates to improvements in seismic energy sources used to create pulses in a liquid medium such as water. More particularly, it relates primarily to improvements in such seismic sources which create pulses in the medium by the detonation of explosive gas in an explosion chamber which communicates directly with the medium.
2. Description of the Prior Art
In prospecting in subsea areas and in other earth formations underlying bodies of water, it is desirable to provide a source of energy for introducing pulses or waves into the water. These waves propagate through the water, enter the underlying formation, are reflected in part by discontinuities in the formation, and subsequently propagate back through the water and are detected by geophones and other measuring devices at or near the water's surface. The characteristics of the reflected waves are compared with the characteristics of the waves at or near their introduction into the water. This comparison reveals valuable information about the structure of the underlying formation and the probability of the presence of petroleum accumulations in the formation.
It has become common to use seismic sources known as gas guns to induce pulses of desired characteristics in the liquid medium. These guns operate by abruptly introducing under high pressure bubbles of compressed air or other gaseous material into the liquid medium, thereby generating a pulse in the medium, or by abruptly expanding a flexible member in contact with the medium, thereby generating a pulse in the medium. An example of the former type of gun is shown in U.S. Pat. No. 3,397,755 (1968) to Loper; an example of the latter type of gun is shown in U.S. Pat. No. 3,658,149 (1972) to Neal et al.
This invention is directed to guns of the former type, and in particular, primarily to guns which achieve the abrupt introduction of high pressure gas into the medium through the detonation of explosive gas in an explosion chamber which communicates with the medium. U.S. Pat. No. 3,397,755 illustrates a subsea seismic source in which a valve is interposed between the chamber of the gun and the liquid medium and the release of the gas to the medium is controlled by the valve. It is also well known in the art to make and use open ended gas guns in which no valve is interposed between the chamber and the medium, but instead the chamber communicates directly with the medium through a section of open pipe or other outlet which remains open continually. Such open ended gas guns have certain advantages of simplicity of construction, operation and maintenance over gas guns with valves which control the release of the gas into the medium. However, an open ended gas gun can have the disadvantage that in operation, as it is moved through the liquid medium, eddy currents are created in the medium, so that the medium enters the explosion chamber and interferes with the introduction and detonation of the explosive gas.
A further disadvantage of gas guns, whether open ended or not, is that the detonation or release of the gas and subsequent abrupt introduction of the gas into the medium can create a recoil of the barrel of the gun. This recoil, depending on the mass of the barrel, the magnitude of the detonation and other factors, may be damaging to the equipment and dangerous. Leonard proposes in U.S. Pat. Nos. 3,588,801 (1971) and 3,951,231 (1976) eliminating or reducing the recoil by closing the end of the barrel and providing a series of side ports to permit the discharge of the gas into the medium. While this technique may reduce or eliminate the recoil, it also greatly reduces the efficiency of the transfer of energy to the medium.